1. Field of the Invention
The present invention generally relates to wobble signal detection circuits and optical disk apparatus having such, and more particularly, to a wobble signal detection circuit for detecting an Absolute Time In Pregroove (ATIP) signal from an optical disk such as a compact disk recordable (CD-R) in an optical disk apparatus, and an optical disk apparatus having such.
2. Description of the Related Art
Optical disks of a direct-read-after-write type include two types: write-once and erasable. With respect to a write-once optical disk, data is written thereon mainly by focusing a light beam into a spot on a data recording surface of the disk made from tellurium (Te) or bismuth (Bi) so as to form a pit in the disk at the location of the spot, or by focusing a light beam into a spot on a data recording surface of the disk made from Sb2Se3, TeOx or a thin film of organic dye so as to alter reflectivity of the disk at the location of the spot.
A CD-R, which is a write-once optical disk, includes a number of pregrooves as guiding tracks. The pregrooves radially wobble slightly at a center frequency of 22.05 kHz. Address information during recording called ATIP is multiplexed and recorded in the pregrooves by frequency shift keying (FSK) with a maximum deviation of ±1 kHz.
During recording of data onto and reproduction of data from the CD-R, a wobble signal having the above-mentioned center frequency of 22.05 kHz is reproduced from the pregrooves to detect the ATIP information, which is employed to confirm a recording position on the data recording surface of the CD-R during recording.
In other words, the ATIP information is detected in the following three modes: a first mode wherein a wobble signal is reproduced from an unrecorded CD-R, a second mode wherein a wobble signal is reproduced from a CD-R during the recording, and a third mode wherein a wobble signal is reproduced from a recorded CD-R.
FIG. 2 shows a pregroove 10 of a CD-R and a 4-part detector 12 (a photodetector) for detecting a light beam reflected back from the pregroove 10. The 4-part detector 12 is made of four detector parts 12A, 12B, 12C and 12D. In FIG. 2, the detector parts 12A and 12B form the left half and the detector parts 12C and 12D form the right half of the 4-part detector 12 with respect to a scanning direction indicated by an arrow. Conventionally, in the above-mentioned second mode, when the light beam is focused into a spot on the pregroove 10, the detector parts 12A, 12B, 12C, and 12D detect the reflected beam, and output detection signals A, B, C and D, respectively.
The detection signals are sampled and held. Then, the detection signals A and B detected by the respective detector parts 12A and 12B are added, and the detection signals C and D detected by the respective detector parts 12C and 12D are added.
The signal (C+D) is subtracted from the signal (A+B) so that an output signal (A+B)−(C+D) is obtained. Then, the output signal (A+B)−(C+D) is compared with a reference voltage, so that a binary wobble signal is obtained.
FIG. 3A shows a waveform of the output level of the detection signal A. As shown in FIG. 3A, the power of the light beam alternately repeats a write power state (the maximum value) and a read power state (the minimum value). A portion of the waveform of FIG. 3A including lower peaks is shown on an enlarged scale along the time-base direction in FIG. 3B. When the light beam is in the read power state, the detection signal A shown in FIG. 3B is sampled and held at sampling timings corresponding to rising edges of a sampling pulse signal shown in FIG. 3C. The sampled and held signal includes a noise generated by the above-described sampling (a sampling noise), so that the signals (A+B) and (C+D) have waveforms shown in FIGS. 3D and 3E, respectively. The sampling noise includes a noise generated by the swing of a read light caused by the transition of the beam power state from the write power state to the read power state. Moreover, in the case of overwriting an erasable optical disk such as a compact disk rewritable (CD-RW), the sampling noise further includes a signal component previously recorded on the disk, which appears as a noise in the sampling. The subtraction of the signal (C+D) from the signal (A+B) is performed to offset the sampling noises so that the output signal (A+B)−(C+D) has a desirable waveform with reduced sampling noise as shown in FIG. 3F. Thereafter, the output signal (A+B)−(C+D) is compared with a reference voltage, so that the binary wobble signal is obtained.
However, the gain and offset of a sample-and-hold circuit are inconsistent among the individual circuits, and cause errors in the output level of the sample-and-hold circuit. Therefore, the waveforms of the sampled and held detection signals A, B, C and D include the sampling noises having different levels. For example, when the sampling noise of the signal (A+B) shown in FIG. 1A has a noise level higher than that of the sampling noise of the signal (C+D) shown in FIG. 1B, the sampling noises are not completely offset by the subtraction. Therefore, the sampling noise remains in the output signal (A+B)−(C+D) as shown in FIG. 1C. As the sampling is performed at higher speed, the noise level of the sampling noise becomes higher. Therefore, when a recording speed becomes so high that the sampling noise cannot be ignored, there arises a problem of deterioration in performance of reproducing the ATIP information from the wobble signal.